In 2009, Saguaro National Park was declared one of America’s 25 national parks most imperiled by climate change (Saunders et al. 2009). Climatologists have predicted hotter and drier conditions in the Southwest (Barnett et al. 2004; Seager and Vecchi 2010), increasing the urgency for the park not only to understand impacts of climate change on natural resources but also to better communicate them to the public. Volunteer citizen scientists regularly help resource managers at the park efficiently gather large amounts of field data. At a time when there is much confusion and misinformation about climate change, this type of hands-on participation also has educational value by helping demystify science. In 2010, volunteers played a major role in the Saguaro Census, a monitoring program designed to study long-term ecological change in the park.

Saguaros (Carnegiea gigantea) are large columnar cacti that can live more than 200 years. Beloved by Arizona residents and visitors, they are also well studied. One of the longest annual monitoring programs for any species in a national park occurs at Saguaro, where some plots have been monitored for 70 years (and currently by researchers Tom Orum and Nancy Ferguson).

Saguaro National Park was established as a national monument in 1933 to protect the magnificent stand of large saguaros, known as the “Cactus Forest,” in the Rincon Mountains east of Tucson. The Tucson Mountain District, west of Tucson, was added in 1962, and after further expansion in 1994, the monument became a national park. Even in the 1930s many older giants were observed to be dying, and few young saguaros could be found. The decades-long decline of the Cactus Forest has been dramatically captured in repeat photographs compiled by saguaro researcher Ray Turner (fig. 1). In 1962, researchers predicted that the species would disappear from the park by 2000.

Although many factors influence saguaro recruitment, growth, and survival, research reveals that climate can be a major driver of population change. The saguaro is a subtropical plant that tolerates frost but not freezing, and severe freeze events in the 1970s are believed to have been the proximate cause of mortality of many older and very young individuals (Steenbergh and Lowe 1983). Though adults have a high tolerance for extreme heat and drought, young saguaros are very sensitive to these factors; thus recruitment appears to be episodic, coinciding with cooler, wetter periods (Drezner and Balling 2002). In addition, as a desert plant the saguaro is not fire-adapted, which may limit its distribution at higher elevations where flammable grasses are more abundant. Because minimum winter temperatures have become warmer in the Sonoran Desert during the past few decades (Weiss and Overpeck 2005), and climate models predict future decreased winter rainfall (Seager and Vecchi 2010), climate change may thus have a complex influence on saguaros—positive in response to warmer winters, but negative because of increased drought. A particular concern is that warmer winters are believed to promote an invasive African grass, buffelgrass (Pennisetum ciliare), that competes with saguaros and promotes fire in a desert ecosystem dominated by plants (like saguaros) that have not evolved with fire (Stevens and Falk 2009).

Although many factors influence saguaro recruitment, growth, and survival, research reveals that climate can be a major driver of population change.